The present disclosure generally relates to shell molds for directional casting, and more particular, to high emittance shell mold compositions that provide a high thermal gradient.
In the manufacture of components, such as nickel based superalloy turbine blades and vanes for turbine engines, directional solidification (DS) investment casting techniques have been employed in the past to produce columnar grain and single crystal casting microstructures having improved mechanical properties at the high temperatures encountered in the turbine section of the engine.
For directional solidification of superalloys, the solid-liquid interface needs a high thermal gradient to yield good cast microstructure. In order to provide a high thermal gradient, heat needs to be removed from the solid casting. However, during the casting process, the metal shrinks away from the mold after the metal solidifies upon cooling; thus, the heat must radiate across an air gap from the surface of the metal to the surface of the mold, from where it can be conducted away. The shrinkage associated with solidification and cooling is a consideration for many casting processes as it affects the casting dimensions and the formation of hot tear cracks as well as contributing to other defects. In continuous casting processes, the molds are often tapered to account for the shrinkage but generally require a fundamental understanding of the shrinkage phenomena during the solidification and cooling of a solidifying shell.
Conventional mold ceramics are selected for strength and chemical inertness. For directional solidification of superalloys, the mold material is typically selected from quartz, fused silica, zircon, alumina, aluminosilicate, and yttria. Typically the process for forming the molds includes dipping a wax pattern into a slurry comprising a binder and a refractory material, so as to coat the pattern with a layer of slurry. The binder is often a silica-based material. Colloidal silica is very popular for this purpose, and is widely used for investment-casting molds. Commercially available colloidal silica grades of this type often have a silica content of approximately 10%-50%. Oftentimes a stucco coating of dry refractory material is then applied to the surface of the slurry layer. The resulting stucco-containing slurry layer is allowed to dry. Additional slurry-stucco layers are applied as appropriate, to create a shell mold around the wax model having a suitable thickness. After thorough drying, the wax model is eliminated from the shell mold, and the mold is fired.
Sometimes, before the shell has cooled from this high temperature heating, the shell is filled with molten metal. Alternately, the mold is cooled to room temperature, and is stored for later use. Subsequent re-heating of the mold will be controlled so as not to cause cracking. Various methods have been used to introduce molten metal into shells including gravity, pressure, vacuum and centrifugal methods. When the molten metal in the casting mold has solidified and cooled sufficiently, the casting may be removed from the shell.
Facecoats are sometimes used to form a protective barrier between the molten casting metal and the surface of the shell mold. For example, U.S. Pat. No. 6,676,381 (Subramanian et al.) describes a facecoat based on yttria or at least one rare earth metal and other inorganic components, such as oxides, silicides, silicates, and sulfides. The facecoat compositions are most often in the form of slurries, which generally include a binder material along with a refractory material such as the yttria component. When a molten reactive casting metal is delivered into the shell mold, the facecoat prevents the undesirable reaction between the casting metal and the walls of the mold, i.e., the walls underneath the facecoat. Facecoats can sometimes be used, for the same purpose, to protect the portion of a core (within the shell mold), which would normally come into contact with the casting metal.
The solidification rate of the molten metal in an investment casting mold significantly affects the microstructure, strength, and quality of the casting. If the solidification rate is too rapid, the metal may not have enough time to feed liquid metal to accommodate the shrinkage on solidification, resulting in porosity. If the solidification rate is too slow, the casting may exhibit a coarse microstructure. Applicants have discovered that these drawbacks, as well as others, may be avoided or minimized by controlling the cooling rate of the molten metal in an investment casting mold.
Accordingly, there remains a need for molds having high heat emittance so as to provide good cast microstructure.